The basic principle underlying solar-thermal electricity generation (concentrated solar power—CSP) is the following: energy from the sun either directly or indirectly heats water, which vaporizes, and the resulting steam drives a turbine whose motion is converted into electricity in a generator. One advantage of the power generated by CSP units is that it is completely CO2 free, and therefore has a negligible carbon footprint.
In order to achieve the high temperatures required to heat the water, the solar radiation must be concentrated. CSP plants concentrate solar energy using mirrors distributed across a small area. The four major CSP technologies include parabolic troughs, linear fresnel, power towers, and dish/engine. Parabolic trough collectors represent the most advanced technology for concentrating solar energy. These troughs are typically more than 1,300 feet in length and are made up of parabolically shaped mirror segments. The curvature of the mirrors allows them to concentrate the sun's direct beam radiation onto a linear receiver.
Current parabolic trough power plants use glass mirror panels. The mirrors are typically second-surface silvered glass mirrors, which means that the reflective metal layer, preferably a silver layer, is on the backside of the glass. The glass is typically a 4-millimeter-thick, special low iron, or white glass, with a high transmittance. These mirrors will frequently have a solar-weighted specular reflectivity of about 93.5%. Heretofore, a special multilayer paint coating was commonly used to protect the reflective metal layer on the back of the mirror. In a typical configuration, each mirror panel is approximately 2 square meters in area.
The mirrors on most CSP systems are made of silvered glass because of silver's high solar reflectivity (93%), relatively low cost, and high durability. The most common construction technique involves laminating a thin, silver mirror to a heavier glass backing structure. Other materials that have been used as the reflectors in solar concentrators include silvered polymer films and augmented aluminum reflectors. The reflective layer may also include a copper back layer for long term durability.
Both the silver reflective layer and copper back layers are prone to air oxidation. Moisture can also degrade these metal-containing layers, as water facilitates the corrosion. In particular, such external elements can cause the degradation and destruction of the silver or other metal contained in the reflective layer over time, as evidenced by tarnishing, discoloration, breakdown, and delamination, resulting in the loss of the mirror's reflectivity. Thus, as appreciated in the current state of the art, the metal layers have heretofore been commonly protected with at least two paint layers plus a UV/moisture protection layer backing the paint layers. The paints typically used for this purpose are lead-based paints.
Applicants have come to appreciate an incentive to remove the paint, and in particular the lead-containing components, completely for environmental reasons. High cost is also an issue for multi-layer coatings that have been heretofore used. Therefore, applicants have come to appreciate a need and to formulate a desire to replace the multiple paint layers heretofore commonly used with a layer of a single coating or film that can provide the barrier protection. The present invention addresses these needs and desires, among others.